Water cooling does not actually directly expose Electronics to water. It works very similar to a cars cooling system except water is not toxic while most car coolant liquids are toxic.
It can be quite reliable and quiet for very hot or overclocked computers since there are no fans to clean, but if the water pump dies then the PC will eventually overheat from the water temperature rising from not getting the water cooled by a water cooling device.
Plus, if the plastic pipes crack or become loose then water will leak on to the electronics.
I think Passive cooling with large copper heatsink for the CPU and GPU are the most reliable since it does not require a fan which can break, so all you need to do is blow out the dust from the metal heatsinks, or the power supply and case fans. Plus, it is quiet.
Usually the same type of person which overclock/speed up their PC's Proccessor like a P4 or Celeron, RAM and video card to a faster speed then it is designed to run at with regular fan cooling.
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They don't have much to lose since their PC is already old and slow, so they might as well try to make it run faster by overclocking and use water cooling to deal with the extra heat from modding and overclocking their PC.
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